Prostaglandins are derived from the oxygenation of arachidonic acid by prostaglandin synthases. Prostaglandins mediate a wide variety of physiological actions, such as vasomotricity, sleep/wake cycle, intestinal secretion, lipolysis, glomelular filtration, mast cell degranulation, neurotransmission, platelet aggregation, leuteolysis, myometrial contraction and labor, inflammation and arthritis, patent ductus arteriosus, cell growth and differentiation (Coleman, R. A., Smith, W. L., and Narumiya, S. 1994. Pharmacol. Rev. 46: 205-229; Goetzl, E. J., An, S. and Smith, W. L. 1995. FASEB J. 9:1051-10585). Prostanoids mediate their actions through binding to distinct receptors which belong to the super family of rhodopsin-like seven transmembrane helical receptors. These receptors are coupled to heterotrimeric G-proteins comprised of α, β and γ subunits which, upon activation, elicit alterations in cell calcium, initiate phosphoinositide hydrolysis or promotion or repression of cyclic adenosine monophosphate synthesis (Strader C. D. et al., 1994 Ann. Rev. Biochem. 63: 101-132).
Of the five pharmacologically distinct prostanoid receptors for PGE2, PGI2, PGD2, PGF2α and TxA2 and their many isoforms, the receptor for PGF2α, also called FP receptor, shows limited tissue distribution, is predominantly expressed in corpora leutea, uterine myometrium, trabecular meshwork of the eye, and to a lesser extent in vascular smooth muscle. Initiation of labor is marked by a tremendous rise in PGF2α levels and increased uterine contractility. The wide spread use of PGF2α analogues to induce labor in veterinary industry points to the primary role of PGF2α and its receptor in parturition. This is underscored by the fact that mice lacking the FP receptor fail to undergo labor (Sugimoto et al., Science, 277: 81-83, 1997). In the face of escalating costs incurred as a result of premature births and associated complications to the neonate, such as intraventricular hemorrhage, bronchopulmonary displasia and periventricular leukomalacia leading to cerebral palsy, prolongation of gestation by arresting premature labor is an effective preventive therapy. The relative success of nonsteroidal anti-inflammatory drugs as a short term therapy toward prevention of premature labor is based on their inhibitory actions upon the synthesis of prostaglandins, particularly PGE2 and PGF2α. However, inhibition of the former is associated with serious complications to the fetus such as the closure of ductus arteriosus, renal failure and pulmonary hypertension. Hence there is a therapeutic need for finding antagonists of FP receptor to treat premature labor.
At another level, PGF2α has been attributed to a major role in dysmenorrhea, a condition which afflicts 5%-7% of premenopausal women. A pre-menstrual increase in PGF2α levels resulting in myometrial spasms underlies the pathogenesis of this disorder. Lack of effective antagonists of FP receptor for extended therapy hampered the advances in preventing premature labor and associated sequelae, and the provision of such antagonists is the subject of this application.
Human FP receptor is a 45 kDa integral membrane glycoprotein, consisting of 359 amino acids and shares only 47% sequence identity with EP1 receptor, and to a lesser extent with other prostanoid receptors (Abramovitz et al. 1994. J. Biol. Chem. 269: 2632-2636). Binding of PGF2α to FP receptor is followed by the activation of the Gαβγ complex, increased GTP binding by the Gα subunit, stimulation of phospholipase Cβ activity, release of inositol phosphates, increased intracellular calcium and subsequent signal transduction phenomena ultimately leading to smooth muscle contraction (Coleman, R. A. et al. 1994. Pharmacol. Rev. 46: 205-229). Since the natural ligand, PGF2α and the ligand-based compounds have cross-reactivity with other prostanoid receptors and to date, no effective and selective antagonists of FE receptor have been disclosed, it is of immediate therapeutic relevance in preterm labor and dysmenorrhea to provide FP antagonists, as is done in this present invention.
Modification of the natural ligand of FP receptor, PGF2α, yielded potent and selective agonists of the receptor, however selective and potent antagonists were not disclosed by these approaches. Two compounds, Phloretin (Kitanaka J et al 1993 J Neurochem 60: 704-708) and AL8810 (Griffin B W et al 1999. J. Pharmacol. Exp. Ther. 290 (3): 1278-1284) have been shown to have antagonistic activity to FP receptor. Phloretin, also shown to have antagonistic effects on glucose transport (Lefevre P G 1961. Pharmacol Rev 13: 39-70) is a weak antagonist of FP receptor (IC50 20 μM) and nonselective with respect to PGE2. AL8810, an 11-fluoro 15(2-indanyl) derivative of PGF2α, is shown to be a weak partial agonist of FP receptor, even though it is found to selectively antagonize FP receptor in the presence of a fluprostenol, a full agonist of FP receptor (Griffin B W et al 1999. J. Pharmacol. Exp. Ther. 290 (3): 1278-1284).
Hence there is need to provide highly selective and potent antagonists to FP receptor with a view to develop therapeutic formulations to arrest premature labor and dysmenorrhea. Most importantly, embodiments of the present invention contain inhibitors of FP receptor, and demonstration of the inhibitory action of said peptides and their peptidomimetics on the biological activity of FP receptor. Exemplary embodiments include the utility of the peptide and peptidomimetic inhibitors for reducing the intensity of uterine contraction, said contraction being a central mechansim involved in the initiation and progression of labor as well as menstrual pain.